Tubes Overheating?

[JumpMarine]

Quick question about my JCM 600. Anyone ever have a problem of overheating an amp? Here's what happened tonight. Played for about hour and half, on the gain channel at about 50%, vol at 100%, FX loop 100% and MV at 30 to 50%. During the last 20 minutes of playing I began to get unwanted feedback and microphobic scratching sound. I then switched over to the clean channel and it still had the microphobic sounds for about 10 minutes. My buddy continued to play for about a half hour and the sound quality came back after the tubes cooled down a bit. Is this normal? Should I run a fan behind the amp during extended playing? Sorry to ask so many questions, but I love this amp and hope that there isn't something wrong with it.

[59paul]

Re: Tubes Overheating?

I always run a fan behind my JCM 800's. I did with my JCM 600, also. How long has it been since it's been re-tubed? The microphonics could be the pre-amp tubes wearing out. I'm pretty sure Marshall quit making the 600 around the late 90's. If it has the original pre's in it, it could be time to replace em. If you do, I'd go ahead and replace all the tubes. That way, you have a freshly re-tubed amp. Also, the 600 has vents on top, where the handle is, so that will allow some heat to escape, which helps prolong your tube life. I'd start by re-tubing, and if the problem persists, see a tech. My 600 got good mileage as far as tubes were concerned, and mine never over heated. It's not too bad cost wise to re-tube it, if I remember correctly. Hope this helps.

[Curly]

Re: Tubes Overheating?

you can find a microphonic tube by tapping on them with your fingernail, or something like a pencil (not hard, of course)

or try replacing them one at a time with a known good tube to find a bad one

[59paul]

Re: Tubes Overheating?

Hey Jump, that's some good advice from Curly. Try replacing the pre's one at a time with a " good " tube, and see if the problem goes away.

[JumpMarine]

Re: Tubes Overheating?

I just retubed about 3 weeks ago with Svetlana el34's and jj 12ax7's. I'm not sure if "microphobic" is the right way to describe what I am hearing. The sound is similar to the faint static that a scratchy pot makes when you turn it. But in this case, I am simply playing and it makes the sound. At first I thought that it was a loose cone on the speaker so I swapped it for the same one in the ext cab. All was well until it was pushed hard for an hour and a half. The sound also began to get muffled, like the treble was turned down. This only lasted for a few minutes and went away. JeffH pm'd me and suggested running a fan on the back. I think that I will try that first then try to elliminate a possible bad preamp tube. Thanks guys!

[59paul]

Re: Tubes Overheating?

It's possible that even though ya put new tubes in it recently, you could have gotten a bad one.